It is known to react chromium ore by roasting with alkali compounds, optionally in the presence of leaning materials, to alkali chromates at temperatures of 900.degree. to 1100.degree. C. The reacting takes place conventionally in revolving tubular kilns heated directly by means of hot flame gases. The reaction proceeds, depending on the alkali compound used, mainly according to the following overall reaction equations (1), (2) and/or (3): EQU 4 FeCr.sub.2 O.sub.4 +8 Na.sub.2 CO.sub.3 +7 O.sub.2 .fwdarw.8 Na.sub.2 CrO.sub.4 +2 Fe.sub.2 O.sub.3 +8 CO.sub.2 ( 1) EQU 4 FeCr.sub.2 O.sub.4 +16 NaOH+7 O.sub.2 .fwdarw.8 Na.sub.2 CrO.sub.4 +2 Fe.sub.2 O.sub.3 +8 H.sub.2 O (2) EQU 4 FeCr.sub.2 O.sub.4 +16 NaHCO.sub.3 +7 O.sub.2 .fwdarw.8 Na.sub.2 CrO.sub.4 +2 Fe.sub.2 O.sub.3 +16 CO.sub.2 +8 H.sub.2 O (3)
A limiting factor for the rate of reaction and hence the space-time yield, if the reaction is carried out on a large industrial scale, is the rate of access of the oxygen to the reaction mixture consisting of chromium ore, alkali compounds and optionally leaning materials.
On the one hand the oxygen content of the kiln atmosphere resulting from the mixing with the flame gases of the direct heating is in general limited to only 8 to 12%. 0n the other hand the reaction mixture releases CO.sub.2 and/or water vapour, which escapes out of the bed of the reaction mixture in counter-flow to the oxygen from the kiln atmosphere.
In order to increase the rate of reaction it has already been proposed according to U.S. Pat. No. 3,733,389 that oxygen be supplied additionally at the outlet end of the revolving tube (outlet of the reaction mixture) via nozzles directed onto the bed surface. This measure has proved to be not very effective, since the additionally supplied oxygen is drawn and discharged through the kiln mainly in the form of streaks together with the flue gases.